1. Field of the Invention
The present invention relates to internal combustion engines and more particularly to methods and systems for controlling combustion modes used in the various cylinders of such engines.
2. Background of the Invention
As is known in the art, an engine may be designed to operate in one or more combustion modes. There are combustion modes providing operation over a wide range in torque and speed. There are other combustion modes, which, while used to operate in a narrower range of torque and speed, have other desirable characteristics, such as low emissions, high fuel efficiency, or smooth combustion. To exploit the advantages of the combustion modes having the limited operating range, it is known in the art to operate the engine in the limited range (LR) combustion mode when possible, and switch to a wide range (WR) combustion mode having the wide range of speed and torque when dictated by demand on the engine.
More particularly, when an operator demanded engine torque is within the operating region for a narrow range of speed and torque, all cylinders operate in the LR combustion mode. When operator demanded torque is outside of the LR combustion mode, all engine cylinders are operated according to a wide range (WR) combustion mode. As mentioned above, the LR combustion mode has desirable combustion characteristics, e.g., low emissions, high fuel efficiency, quiet combustion and these characteristics are used only when the engine operates within the operating limits of the LR combustion mode. Once torque demand exceeds the capability of the LR combustion mode, the engine switches all cylinders to the WR combustion mode.
The inventors have recognized a method for controlling a multi-cylinder internal combustion engine by operating a first group of cylinders according to a first combustion mode and operating a second group of cylinders according to a second, different combustion mode with the total torque provided by said first and second groups substantially equals an operator demanded engine torque. As an example, the first combustion mode is a LR combustion mode and the second combustion mode is a WR combustion mode with the torque provided by the LR combustion mode being less than operator desired torque and the torque provided by the WR combustion mode being greater than operator desired torque. Thus, the cylinders operating in the WR combustion mode make up the deficit of cylinders operating at the LR combustion mode to thereby result in the effect of all cylinders meeting operator demand. The benefit of the approach is that the desirable operating characteristic, e.g., fuel efficiency, of the LR combustion mode is achieved in the first group of engine cylinders when operator demanded torque is outside of its normal operating range thereby improving these characteristics compared with those characteristics in an engine having all cylinders operating with the WR combustion mode.
In accordance with the invention, a method is provided for controlling an internal combustion engine having a plurality of cylinders. The method includes operating a portion of the cylinders according to a first combustion mode and operating a second group of the cylinders according to a second, different combustion mode.
The method further includes operating the plurality of cylinders to produce from the engine a net torque substantially equal to an operator demanded engine torque.
In one embodiment, one of the combustion modes is a homogeneous-charge, compression-ignition combustion mode in which fuel and oxidizer are substantially homogeneously mixed prior to ignition and ignition occurs via autoignition and the other one of the combustion modes is a homogeneous-charge, spark-ignition combustion mode in which fuel and oxidizer are substantially homogeneously mixed prior to ignition and ignition occurs via a spark. A primary advantage of this embodiment is that the fuel economy benefit of HCCI is extended.
In one embodiment, the second mentioned one of the combustion modes is a heterogeneous-charge, compression-ignition combustion mode in which fuel and oxidizer are mostly unmixed at the time of ignition and ignition occurs via autoignition. An advantage of this embodiment is low levels of regulated emissions compared with a system in which heterogeneous-charge, compression-ignition is used solely.
In one embodiment, the first combustion mode is a direct-injection, stratified-charge combustion mode in which fuel and oxidizer are mostly unmixed at the time of ignition and ignition occurs via spark ignition. The second combustion mode is a homogeneous-charge, spark-ignition combustion mode in which fuel and oxidizer substantially homogeneously mixed prior to ignition and ignition occurs via spark ignition. A primary advantage of this embodiment is a fuel efficiency advantage.
In accordance with another feature of the invention, a method is provided for controlling torque in an internal combustion engine having a plurality of cylinders. The method includes operating the plurality of cylinders according to a first combustion mode, such engine operating to produce a torque within a range of torque. In response to a demand for a change in torque to a torque outside of such range, the engine transitions from operating the engine with the plurality of cylinders in the first combustion mode to operating such engine with a one portion of such plurality of cylinders remaining in the first combustion mode while a different portion of the cylinders transitions operate in a different combustion mode.
In one embodiment, the net torque provided by engine cylinders substantially equals the demanded torque.
Yet another embodiment of the present invention is a method for controlling torque in a multi-cylinder internal combustion engine in which a first portion of engine cylinders operate at a first combustion mode and a second portion of cylinders operate at a second combustion mode, which is different than the first combustion mode. In response to a demand for a change in torque, the second portion of engine cylinders transition from the second combustion mode to the first combustion mode.
Another advantage of the present invention is that when operator demanded torque calls for a torque trajectory, which goes outside of the torque range for the LR combustion mode, the present invention allows for a smoother transition out of the LR combustion mode. That is, if the engine is operated at the LR combustion mode and a higher torque is demanded, all of the cylinders are changed to a WR combustion mode, a portion of engine cylinders transition to the WR combustion mode while the remaining cylinders continue to operate in the LR combustion mode, thereby easing the transition process. If a further increase in torque is required in which none of the cylinders can operate in the LR combustion mode and still provide desired torque, only the portion operating under the LR combustion mode makes a transition.
According to yet another embodiment for controlling an internal combustion engine is a system having an electronic control unit operably connected to engine cylinders. The electronic control unit commands a first portion of cylinders to operate at a first combustion mode and a second portion of cylinders to operate at a second combustion mode. In response to a demand for a change in torque, the electronic control unit commands a transition to operating the engine with a third portion of the cylinders in the first combustion mode and a fourth portion of the cylinders in the second combustion mode; the number of cylinders in the first and third portions of cylinders being unequal. The first combustion mode is either a limited range combustion mode or a wide range combustion mode and the second combustion mode is the other, i.e., different from the first combustion mode. An advantage of this aspect of the present invention is that a transition can be made on a cylinder-by-cylinder basis. As an example, a six-cylinder engine operating with a 1:5 ratio of cylinders in the first:second combustion modes can transition through the following steps: 2:4, 3:3, 4:2, and 5:1. In this way, transitions are accomplished in a more seamless and less abrupt manner.
The above advantages, other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.